Preheat oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, beat the 1/2 cup softened butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Mix in 1 teaspoon vanilla extract.
In a small bowl, combine mashed bananas, buttermilk, sour cream, and lemon juice.
Add the dry ingredients to the butter mixture alternately with the banana mixture, beginning and ending with the dry ingredients. Mix just until combined.
Divide the batter evenly between the prepared cake pans and smooth the tops.
Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Allow cakes to cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
To make the frosting: In a large bowl, beat the 1 1/2 cups softened butter until creamy.
Gradually add powdered sugar, 1 cup at a time, beating well after each addition.
Add 2 teaspoons vanilla extract and 1/4 cup heavy cream. Beat on high speed for 3-4 minutes until light and fluffy.
If frosting is too thick, add more heavy cream, 1 tablespoon at a time, until desired consistency is reached.
Place one cake layer on a serving plate. Spread a thick, even layer of frosting over the top.
Place the second cake layer on top and frost the top and sides of the cake evenly.
Decorate as desired and refrigerate the cake for at least 30 minutes before serving.